1. Field of the Invention
The present invention relates to a device for the lateral stabilization of the spine, which device is to be implanted along the vertebral column in the region of one or both of its left and right lateral sides, in order to stabilize at least two vertebrae one relative to the other. Such stabilization is desirable especially within the context of the treatment of degenerative or traumatized spine. The invention relates more particularly to the treatment of the dorsolumbar spine, but is likewise applicable to the treatment of the cervical spine.
2. Brief Description of the Related Art
For the treatment of an intervertebral instability, a first known possibility comprises fusing two adjacent vertebrae, which amounts to depriving those two vertebrae of their relative freedom of movement. Totally rigid assemblies are implanted for that purpose in a fixed manner along the spine in order permanently to block the articulating joint between the two vertebrae to be fused. U.S. Pat. No. 6,296,644 accordingly proposes a vertebral assembly constituted by a plurality of vertebral elements which are to be fixed to the same number of vertebrae and which are connected in pairs by “lockable” joints: when the assembly is being fitted, the joints are movable in order to facilitate the relative positioning of the vertebral elements along the spine, and then, when fitting is complete, the joints are fixed permanently by means of rings having shape memory so that, in use, the vertebral elements are completely fixed relative to one another. However, this type of operation of arthrodesis of the vertebrae leads to degeneration of the adjacent disks, on which it is subsequently necessary to operate.
Another known possibility for treatment of the spine comprises operating at an earlier stage than that which involves arthrodesis. A first solution of that type is proposed in DE-U-298 14 320: a plurality of separate vertebral elements, each fixed to adjacent vertebrae, are in use movable relative to one another, while being connected in pairs by rectilinear telescopic joints according to the longitudinal direction of the spine. This movable assembly adapts to a certain development of the kinematic behavior of the spine, for example as it grows, but does not provide actual dynamic stabilization of the vertebrae and accordingly does not prevent, for example, crushing or deformation of the intervertebral disks.
A second solution is proposed in JP-A-10 277070: the posterior sides of two adjacent vertebrae are connected vertically by two pistons, a sleeve of resilient material being inserted between the male part and the female part of each piston. The transverse cross-section of each piston is elliptical, which on the one hand prevents the male and female parts of the piston from being guided in rotation one relative to the other about a vertical axis and on the other hand centres the articulating movements between the male and female parts either in the piston or in a plane that passes through the two pistons, that is to say well behind the vertebrae. The kinematics imposed on the vertebrae is accordingly very different from the normal anatomical behavior of the spine, with considerable risks that the intervertebral disk will be pinched, or even crushed, at least in its anterior portion.
Other solutions aim to implant a lateral device for dynamic stabilization, such as those proposed in U.S. Pat. No. 5,423,816, U.S. Pat. No. 5,704,936 and U.S. Pat. No. 6,616,669. To that end, this type of device comprises, on the one hand, rigid elements that are to be anchored in the bone of the same lateral side of two adjacent vertebrae and, on the other hand, flexible joining elements between the rigid elements. These flexible elements, such as springs or flexible arms, extend laterally along the spine and thus relieve the intervertebral disk by reducing any excess pressure in the region of the articulating surfaces between that disk and the vertebrae. Such devices are more comfortable for the patient because they allow the mobility of the spine to be retained. However, the use of that type of dynamic device is found to be difficult in practice. Dimensioning of the flexibility of the joining elements is difficult because it must be adapted to each patient according to his pathology and morphology and, in the long term, the resilient behavior of those elements changes. If those parameters are poorly controlled, it is not possible to ensure that a kinematics appropriate to the spine is respected, which can lead to poor stabilization of the intervertebral space and to aggravation of the damage that it is desired to treat.